The invention relates generally to a multi-stage vacuum pump installation and, more particularly, to an installation which has an oil-lubricated or dry-running mechanical displacement pump in the final atmospheric stage. The installation also has at least one additional pump located in the vacuum side which is connected to the oil-lubricated or dry-running mechanical displacement type pump.
The efficiency of mechanical displacement pumps in the entire vacuum range is known. DE-OS 35 45 982 and DE-GM 84 27 615 disclose multi-stage vacuum pump installations containing several rotary vane pumps in series. Because such pumps generally require oil as a lubricating and sealing medium, new oil must be continuously supplied in all stages of the installation. Although the supplied amounts of oil are relatively small, the constant or controlled supply of fresh oil is a considerable cost factor. Additionally, the spent oil must first be removed from the transported medium by independent separators and then eliminated.
A multi-stage vacuum pump installation that contains a Roots pump located upstream and connected to a rotary vane pump is disclosed in "Criteria for pump selection for the creation of a vacuum","Maschinenmarkt", vol. 88, No. 17, Mar. 2, 1982, published by Vogel-Verlag Wurzburg. The pistons of Roots pumps rotate contactless and thus Roots pumps are efficient and superior to other mechanical vacuum pumps. Improved overall efficiency of a multi-stage vacuum pump installation can be achieved by placing a Roots pump upstream in the installation. The improved efficiency of multi-stage installations using Roots pumps is only possible for pressure differences of less than 50 mbar. The many narrow gaps of Roots pumps do not permit greater pressure differences because the greater temperature rise connected with higher pressure differences causes thermal expansions which, due to the narrow gaps, may readily lead to jamming of the pistons.
A greater pressure difference in an installation could be obtained without the danger of piston jamming by intensive cooling of the installation or by connecting several Roots pumps in series. However, intensive cooling or series connection would dramatically increase the construction and maintenance cost of the installation. Additionally, the safety of operation would be impaired.
An installation for high vacuum ranges that contains a molecular pump and an oil-sealed rotary pump is disclosed in U.S. Pat. No. 4,090,815. Oil-sealed rotary pumps are used in vacuum engineering for pressure ranges from 1013 mb (760 Torr) to a maximum of 10.sup.-4 mbar (about 10.sup.-4 Torr). Because oil-sealed rotary pumps are unable to produce a high vacuum, usually the oil-sealed rotary pump is preceded in the installation by a molecular pump. Molecular pumps operate on the principle of pulse transmission at solid faces. The molecular pump of the 4,090,815 patent has a rotating circular disk for its pulse transmission that is rotatably mounted in a two-part housing. Each housing part has a spiral delivery groove whose cross-section tapers in the direction of increasing pressure, which corresponds to increasing molecular density. Each of the delivery grooves and the rotating circular disk form a working channel. The rotating disk constitutes the moving wall of the working channel on to which the molecules impinge. Due to the rotation of the disk, a drift velocity is superposed on the isotropic velocity distribution (corresponding to the wall temperature) of the individual gas molecules. This results in flow of the gas molecules and hence pumping occurs.
The present invention is directed to the problem of further developing an installation of the general type described above which decreases oil consumption, thus decreasing the amount of dirty oil, and increases efficiency when compared to known multi-stage vacuum pump installations.